Effects of Electrostatic and π—π Interactions on the Stabilities of Xanthene Dye—4,4′‐Bipyridinium Complexes: Structural Design of a Geared Supramolecular Machine

Xanthene dyes and substituted 4,4′‐bipyridinium salts form well defined 1:1 complexes in solution and in the crystalline state. Electrostatic, charge‐transfer, and π‐π interactions are the driving forces for the formation of these complexes. Electrostatic interactions contribute 1.1–1.3 kcal/mol per ion pair, and interring interactions, i.e., charge transfer and π‐π interactions, contribute 6.2–6.3 kcal mol⁻¹ to the complexes' stabilization. Application of a bis‐bipyridinium host results in enhanced binding of eosin Y, 1, due to increased π‐π and donor‐acceptor interactions and complementary electrostatic attractions. The binding properties of the bipyridinium moiety to eosin Y is controlled by its functionalization with a photochromic azobenzene unit. The association constants of 8‐trans and 8‐cis to eosin Y, 1, are 3000 M⁻¹ and 38000 M⁻¹, respectively. This difference in the association constants of the diad isomers to 1 allows the photo‐controlled reversibly‐geared association and dissociation of the diad to and from eosin Y in the supramolecular assembly.